Flip-cathode Field Emission Vacuum Triode for THz Applications
Paoloni, C; Riccitelli, R; Brunetti, F; Ulisse, G; Di Carlo, A
University of Roma "Tor Vergata"

The potential of THz radiation is impressive in many fields, such as space, security, medical, biology. The lack of THz sources, with adequate output power, small dimensions, good reliability, repeatable performances, environment temperature working, low cost and low weight, is a crucial limitation in the development of the possible applications. The recent advance in micromaching and nano technologies have permitted to design and realize structures overcoming the actual dimensional limitations, and consequently the extension of the working frequency band in the THz range (100-3000 GHz). Further, the introduction of cold cathodes based on CNTs (Carbon Nano Tubes) represents a great advance in electron beam generation. A cold cathode is highly efficient, since does not need heating. Further, it presents instantaneous switch on time, reduced dimensions and resistant to temperature variation. CNTs are the ideal field emitters in Spindt type cathode and are expected to have longer lifetime and less probability of damages than common emitter [1].
In this abstract a CNT integrated vacuum triode structure for broadband amplification, based on a well established CNT growth facilities and in-house technological process (lithography, etching, vacuum sealing) [2, 3] is proposed. The intrinsic simplicity of the triode allows a very effective realization and a good control of the parasitics. An innovative fabrication process for the realization and the assembly of a vacuum triode based on carbon nanotubes (CNTs) as cathode is presented. The proposed technique is explained to avoid the typical problems due to the growth of carbon nanotubes in classical triode structure. The cathode synthesis process requires the strict control of CNT parameters (location, length, density), in order to avoid cathode-gate shorts and large gate currents (low triode transparencies). Besides, it is often difficult to maintain the integrity of the gate material at the high temperatures needed by synthesis processes. The novel flip-cathode triode structure foresees the realization of two separate elements that are the cathode and the integrated anode-grid structure (fig.1) [2]. This flip-cathode configuration has various benefits such as an easier realization process in which it is possible to prevent the occurrence of short circuits between the grid and the CNTs and the possibility to realize the cathode separately from the grid. In addition in this configuration it is possible to use different kinds of emitting materials, grown on a variety of substrates, opening the way to many new applications for THz vacuum triodes. Besides, the metal structures of the grid are not exposed to the high temperature fabrication process of the CNTs. The electrical triode characterization in terms of input and output characteristics, trans-conductance and DC gain confirm the validity of this realization [3]. The reduced dimensions of the triode (tenths of microns) make this structure very promising for applications in the THz frequency band. Further, if higher output power is required many triodes can be arranged in matrix shape to sum their output power. Further studies and experiments are in progress to optimize the performance and better establish the technological process.

REFERENCES
[1] J. Bonard, H. Kind, T. Stockli, L. Nilsson, Field emission from carbon nanotubes: the first five yearsE Solid State Electronics, 45 893, 2001
[2] R.Riccitelli et al., IVEC 2007, Kitakyushu, Japan
[3] F.Brunetti et al., IEEE Electron Device Letters, 29, 1, 111 (2008)